Previously, `BuildCreator#changePrVisibility()` would throw an error if the PR's
visibility was already up-to-date or if the PR directory did not exist (e.g. was
removed). This method was only used from inside `BuildCreator#create()`, which
had already checked for the existence of the directories.
This commit renames `changePrVisibility()` to `updatePrVisibility()` and makes
it more "forgiving" (i.e. it will only throw if both public and non-public
directories exist). This allows it to be used on events that may or may not have
caused the PR's visibility to change (e.g. a GitHub webhook triggered whenever a
PR's labels change).
5a417b8514